1. Field of the Invention
This invention relates to systems for providing remote readout of data indicating devices and, more particularly, to a remote reading system including apparatus for transmitting digital data from one location to another over existing electrical power lines.
2. Description of the Prior Art
Various systems have been proposed in the prior art for obtaining information from a plurality of remote sources. For example, remote reading systems have been employed to effect the readout of a plurality of utility meters from a central interrogation station which may be mobile or stationary.
In such systems, interrogate data signals generated at a central location are transmitted to a plurality of data indicating devices at locations remote from the central location to effect readout of information provided at such locations. Each data indicating device has associated therewith a transponder for receiving the interrogate data signals and for generating reply data signals representing the information provided by the indicating device for transmission back to the central location.
Some prior art remote reading systems employ RF communication links to enable the transmission of data between the central location and the transponders associated with indicating devices at the remote locations. Such systems require an RF signal antenna at the central location as well as at the location of the transponders at the remote locations. Since the indicating devices being read out are generally located within a building, the signal antennas at such remote locations must be located outside the buildings to enable reception of interrogate data signals transmitted from the central location and the transmission of reply data signals back to the central location. Accordingly, a separate signal line must be extended from the location of each transponder unit within the building to the location of the antenna at the outside of the building.
In other systems the interrogate data signals are transmitted over communication links established over a telephone line connected between the central location and the transponders at locations remote from the central location. However, such systems can be employed only where a telephone line is available at the location of the indicating device and associated transponder.
Thus, in either of these prior art systems, it is generally necessary to provide a separate data communication line between the locations of an indicating device in a building and a point in the building from which the data can be transmitted to a central interrogate station. In applications wherein a plurality of indicating devices are located at different locations within a given building, the requirement of providing the necessary data communication lines may add considerable cost to the system.
In another known system which employs electrical power lines for readout of data indicating devices, data indicating information provided by the data indicating devices is transmitted over the neutral conductor of the power lines. However, this system requires modification of the existing wiring system by severing of the neutral conductor at the location of each data indicating device to permit the insertion of a resonant circuit which isolates the neutral conductor from earth ground to enable transmission of data signals over the neutral conductor.